This invention relates to a system for positioning one member with respect to another member and, more particularly, to a servo system for aligning a recording head over a desired one of a plurality of tracks on a disk for storing data.
In the data processing field, use is made of rotatable disks having magnetizable surfaces which are magnetized along concentric tracks in accordance with data signals representing the information which is to be stored. The data signals are applied to transducers or heads which move radially on command to specified select tracks to magnetize the disk surface; alternatively, the heads sense the recorded data to reproduce the information. The heads typically are coupled to a carriage which is displaced by an actuator of a servo system to position the head over a desired commanded track.
Recently, disks having high track densities have been developed to increase data storage capacity. For example, track densities have been increased from about 100 tracks per inch to about 200-400 tracks per inch; this increase, therefore, imposes a heavy burden on the servo system to position accurately the transducers over the desired track. In these high density disk systems, if the servo system is inaccurate by a small degree, it may undesirably place the head off center of the commanded track, though this small inaccuracy might be tolerable in a lower track density system.
In a typical servo system, an error signal is generated representing the difference between the track position and the position of the head. This error signal then controls a servo motor which moves the carriage to position the head on center over the track. This process continues until no error signal is generated, thereby indicating that the head is on center.
The primary problem with such servo systems is that inherently there is a lag or phase delay between the time a track position signal is received by the servo system and the time the carriage is moved to position the head on the commanded path. This problem exists because such servo systems, particularly those used in disk drives, have a relatively narrow bandpass of about 150 Hz. or lower. That is, the amplitude of the output signal (e.g., carriage movement) is equal to the amplitude of the input signal (e.g., track position) only up to about the system bandwidth. This is important in disk drive systems because usually the disks are rotated at about 2400 RPM, i.e., 40 Hz., which is the frequency of the track position information. As is known, and as can be shown by a graph illustrating the difference in phase between the output and input of a system having a bandpass of about 150 Hz., the carriage movement will lag the track position input by about 22.degree. at the track position frequency of 40 Hz. From such graph, it also can be shown that such phase lag of 22.degree. results in about a 37% error between the track position and the position of the head after it is moved by the carriage.
The above-mentioned 37% error is significant if one considers the effect that runouts such as wobble have on the ability of a servo system to center the head over a track. Wobble, which is the radial movement of an area on a disk with respect to the disk axis during rotation of the disk, may cause the track centerline to be offset from the head by, for example, 500 micro-inches (the track position). This is known as a repeatable error which occurs at the disk rotation frequency of 40 Hz. and is repeatable in that for each revolution of the disk, such area will follow substantially the same wobble path. This means that with the 37% error due to the bandpass of 150 Hz., the head position will be offset by 500 .times. 37% = 185 micro-inches, which is not satisfactory for the higher track densities.
The prior art has recognized two approaches to reducing such errors, one being to reduce the wobble and the other to increase the bandwidth of the system. Wobble is caused by a number of factors including, for example, 1) the free play or fit of the shaft, rotating the disk, on bearings, 2) the stiffness of the bearings, and 3) the mounting of the disks on the shaft, which can cause eccentric shaft rotation. Wobble can be reduced, but this requires more expensive bearings to increase their stiffness, and more expensive mountings to reduce free play and provide a truer circular shaft rotation. For example, there has been used conically shaped shafts and mating hubs to provide a tighter, more accurate fit.
If the bandpass of the servo system is increased, then the above-mentioned error for a disk rotating at 40 Hz, can be reduced. However, as with the solution to reducing wobble, this requires more expensive components. The system bandpass can be increased but only at the expense of using more massive parts, such as heavier carriages and carriage arms supporting the heads. Moreover, the more massive carriage and arms supporting the heads would require greater power consumption by the servo system electrical components to move them.